Long-term stability and performance characteristics of crystal quartz gauge at high pressures and temperatures

Good long-term stability of high precision quartz pressure sensors is necessary for various applications ranging from pressure transient analysis to permanent monitoring systems for optimal reservoir management in the petroleum industry. A crystal quartz gauge (CQG/sup 1/) is a dual-mode, thickness-shear, quartz pressure sensor that has been used in oil field services for the past 8 years. High accuracy, resolution, and fast response time of this sensor enable a reliable estimate of formation permeability and oil/water interfaces in reservoirs that help reduce the overall cost of oil and gas production. The sensing resonator characteristics can be described in terms of equivalent circuit parameters (motional resistance and capacitance), resonator-Q and the short-term frequency stability of both the B- and C-modes of vibrations at various temperatures. The pressure reading errors of manufactured gauges are less than 8.89 kPa (1 psi) (plus 0.01% of the reading because of the uncertainty of the dead-weight tester). The pressure resolution is better than 20.7 Pa (0.003 psi) over a 1-s gate time. An extremely effective dynamic compensation algorithm yields corrected pressure readings with a very fast response time as short as a strain-gauge-based pressure transducer while retaining the high performance of a quartz gauge. Recent long-term stability tests of CBGs show a negligibly small drift of the order of a few tenths of 1 psi (0.1 psi=689 Pa) at 103 MPa (15 kpsi) and 175/spl deg/C for a period of more than 1 yr. These results confirm that the CQG characteristics exceed the demanding specifications for both the well tests and permanent monitoring systems.